Modern-day electronics require multiple patterned layers of electrically or optically active materials, sometimes over a relatively large substrate. Electronics such radio frequency identification (RFID) tags, photovoltaics, and optical and chemical sensors all require some level of patterning in their electronic circuitry. Flat panel displays, such as liquid crystal displays or electroluminescent displays rely upon accurately patterned sequential layers to form thin film components of the backplane. These thin film components include capacitors, transistors, and power buses. The industry is continually looking for new methods of materials deposition and layer patterning for both performance gains and cost reductions.
Thin film transistors (TFT's) may be viewed as representative of the electronic and manufacturing issues for many thin film components. TFT's are widely used as switching elements in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. A TFT is an example of a field effect transistor (FET) and the best-known example of an FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today's conventional switching element for high-speed applications. For applications in which a transistor needs to be applied to a substrate, a thin film transistor is typically used. A critical step in fabricating a thin film transistor involves the deposition of a semiconductor onto the substrate. Presently, most thin film devices are made using vacuum deposited amorphous silicon as the semiconductor, which is patterned using traditional photolithographic methods. Amorphous silicon as a semiconductor for use in TFT's still has its drawbacks. Thus, there has been active work to find a suitable replacement.
There is a growing interest in depositing thin film semiconductors on polymeric or flexible substrates, particularly because these substrates would be more mechanically robust, lighter weight, and allow more economic manufacturing, for example, by allowing roll-to-roll processing. A useful example of a flexible substrate is polyethylene terephthalate that can be provided as a continuous flexible film. In spite of the potential advantages of flexible substrates, there are many problems associated with polymeric substrates when using traditional photolithography during conventional manufacturing, making it difficult to perform alignment of transistor components across typical substrate widths up to one meter or more. Traditional photolithographic processes and equipment may be seriously impacted by the substrate's maximum process temperature, solvent resistance, dimensional stability, water, and solvent swelling, all key parameters in which polymeric substrates are typically inferior to glass substrates.
There also is interest in utilizing lower cost processes for deposition that do not involve the expense associated with vacuum processing and patterning with photolithography. In typical vacuum processing, a large metal chamber and sophisticated vacuum pumping systems are required in order to provide the necessary environment. In typical photolithographic systems, much of the material deposited in the vacuum chamber is removed. The deposition and photolithography processes and equipment have high capital costs and preclude the easy use of continuous web-based systems.
In the past decade, various materials have received attention as a potential alternative to amorphous silicon for use in semiconductor channels of thin film transistors. The discovery of practical inorganic semiconductors as a replacement for current silicon-based technologies has also been the subject of considerable research efforts. For example, metal oxide semiconductors are known that constitute zinc oxide, indium oxide, gallium indium zinc oxide, tin oxide, or cadmium oxide deposited with or without additional doping elements including metals such as aluminum. Such semiconductor materials, which are transparent, can have an additional advantage for certain applications, as discussed below. Additionally, metal oxide dielectrics such as alumina (Al2O3) and TiO2 are useful in practical electronics applications as well as optical applications such as interference filters.
In addition, metal oxide materials can serve as barrier or encapsulation elements in various electronic devices. These materials also require patterning so that a connection can be made to the encapsulated devices.
There is growing interest in combining atomic layer deposition (“ALD”) with a technology known as selective area deposition (or “SAD”) in which a material is deposited to form a thin film only in those areas that are desired or selected. Sinha et al. [J. Vac. Sci. Technol. B 24 6 2523-2532 (2006)] have remarked that selective area ALD requires that designated areas of a surface be masked or “protected” to prevent ALD reactions in those selected areas, thus ensuring that the ALD film nucleates and grows only on the desired unmasked regions. It is also possible to have SAD processes wherein the selected areas of the surface area are “activated” or surface modified in such a way that the thin film is formed only on the activated areas. There are many potential advantages to selective area deposition techniques, such as eliminating an etch process for film patterning, reduction in the number of cleaning steps required, and patterning of materials which are difficult to etch. One approach to combining patterning and depositing a semiconductor is described in U.S. Pat. No. 7,160,819 (Conley et al) that describes materials for use in patterning zinc oxide on silicon wafers.
A number of materials have been used by researchers as deposition inhibitor materials for selective area deposition. Sinha et al., referenced above, used poly(methyl methacrylate) (PMMA) as a masking layer. In U.S. Pat. No. 7,160,819 (noted above), acetone and deionized water were used, along with other process contaminants as deposition inhibitor materials.
U.S. Patent Application Publications 2009/0081827 (Yang et al.) and 2009/0051740 (Hiroshima) describe the use of crosslinkable organic compounds or polymers, such as organosiloxane polymers, as deposition inhibitor materials, in ALD processes to provide various electronic devices. These crosslinkable materials are generally coated out of organic solvents.
The problem with the processes using organosiloxanes is that such materials are soluble only in aggressive organic solvents. Aside from health and environmental concerns, the use of aggressive organic solvents is difficult in large scale manufacturing processes. Thus, organosiloxanes can be difficult to remove from a substrate after being deposited as deposition inhibitors. There continues to be a need for additional deposition inhibitors that are soluble in a range of environmentally friendly solvents so they can be formulated into “inks” and that are compatible with various patterning techniques used for fabricating various devices using chemical vapor deposition techniques such as ALD.